feat(segcache): allow segment cache expiry of 0 to cache forever

[javi11]

What & why

The segment cache expiry could not be set to 0 to keep decoded segments forever. Two layers blocked it:

• The config slider was capped at min=1.
• segcache.WithDefaults() silently rewrote any expiry <= 0 back to 24h — so even a 0 that reached the manager was clobbered.

The cache engine itself (SegmentCache.Cleanup) already treats <= 0 as "never expire", so the intent existed but was blocked upstream.

Changes

• config (internal/config/manager.go): SegmentCacheConfig.ExpiryHours becomes *int so unset and explicit 0 are distinguishable (mirrors the existing Enabled *bool idiom). Validate() normalizes nil -> 24h, preserving the default for unconfigured installs while an explicit 0 means "forever".
• segcache (internal/nzbfilesystem/segcache/manager.go): drop the ExpiryDuration default from WithDefaults(); the config layer now owns the default, so an explicit "forever" survives.
• setup (cmd/altmount/cmd/setup.go): dereference the pointer with a defensive nil guard.
• serve (cmd/altmount/cmd/serve.go): pointer-safe value comparison (intPtrValue) for the dynamic reload detection.
• frontend (StreamingConfigSection.tsx): expiry slider min=0, renders "Forever" at 0, updated helper text and scale label.

Behavior

ExpiryHours	Result
nil (unset)	24h default — no regression for existing installs
explicit 0	Forever (still bounded by the max-size LRU cap)
N	N hours

Reviewer notes

• The *int pointer is the mechanism that distinguishes "never configured" from "explicitly 0"; Validate() guarantees the frontend always receives a concrete number.
• The 5-min cleanup loop no-ops on expiry when <= 0, but Evict() still enforces the size cap, so "forever" is always bounded by MaxSizeGB.
• Includes minor Biome auto-formatting in a few unrelated frontend files (ProviderStatusTable, HealthPage, QueuePage, HealthTable) picked up by bun run check.

Verification

• go build ./... — clean
• go test ./internal/config/... ./internal/nzbfilesystem/segcache/... — pass
• bun run check — pass